Many documents are known describing processes for the catalytic conversion of (gaseous) hydrocarbonaceous feedstocks, especially methane, natural gas and/or associated gas, into liquid products, especially methanol and liquid hydrocarbons, particularly paraffinic hydrocarbons. In this respect often reference is made to remote locations and/or off-shore locations, where no direct use of the gas is possible. Transportation of the gas, e.g. through a pipeline or in the form of liquefied natural gas, is not always practical. This holds even more in the case of relatively small gas production rates and/or fields. Reinjection of gas will add to the costs of oil production, and may, in the case of associated gas, result in undesired effects on the crude oil production. Burning of associated gas has become an undesired option in view of depletion of hydrocarbon sources and air pollution.
The Fischer-Tropsch process can be used for the conversion of synthesis gas (from hydrocarbonaceous feed stocks) into liquid and/or solid hydrocarbons. Generally, the feed stock (e.g. natural gas, associated gas and/or coal-bed methane, heavy and/or residual oil fractions, coal, biomass) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted in one or more steps over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight compounds comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebullating bed reactors.
The Fischer-Tropsch reaction is very exothermic and temperature sensitive, with the result that careful temperature control is required to maintain optimum operation conditions and desired hydrocarbon product selectivity. Indeed, close temperature control and operation throughout the reactor are major objectives.
Starting up such a process will involve new and regenerated catalyst material. However, catalyst material when new is often more active than when it has achieved a steady state activity under reaction conditions. In chemical reactions such as the Fischer-Tropsch reaction, which is very exothermic and temperature sensitive as mentioned above, a higher level of activity of a catalyst at the start up of a reactor is a significant concern.
There is thus required a way of using the initial greater activity of new catalyst material until the reaction process reaches a steady state. Several start-up procedures have been proposed in the prior art to cope with the initial greater activity of the catalyst.
In WO 03/068715 for example is described a process for starting up a Fischer-Tropsch slurry reactor wherein an initial charge of molten wax is established in a Fischer-Tropsch reactor. The reactor contains a portion of its steady state catalyst inventory in contact with the molten wax. The catalyst is supplied to the reactor in the form of a slurry of molten wax and catalyst particles. The reactor preferably contains clean molten wax without catalyst particles and a slurry of molten wax and catalyst particles is then mixed with the clean wax. At start-up, syngas at a flow rate below the steady state flow rate and a H2/CO ratio above the steady state ratio is contacted with the catalyst at a temperature below the steady state temperature.
In WO 2005/026292 and WO 2005/026293 is disclosed a method for start-up of a hydrocarbon synthesis process in a slurry bubble column. The start-up method comprises a specific procedure for charging the catalyst particles via a charging vessel into the conversion reactor. At the end of the charging phase, the reactor is kept at a temperature ranging from 150 to 220° C. and a pressure ranging from 1 to 10 bar and is continuously fed with inert gas to prevent catalyst sedimentation. During a conditioning phase, the temperature is brought to values suitable for conditioning, the inert gas is gradually substituted by synthesis gas up to a concentration ranging from 5-50 vol % and this concentration is maintained for 24-72 hours. Then, the pressure and temperature are gradually increased up to steady state regime values and the concentration of inert gas gradually reduced to zero.